Permanent waving compositions heretofore most extensively used consist predominantly of thioglycolic acid which has a peculiar unpleasant odor. It is also known that thioglycolic acid acts intensively on the human hair, that it is therefore liable to break or remove the hair or reduce the luster of the hair, and that causes a rash on the scalp or damage to the skin and to hair roots. In recent years, cysteine which has no unpleasant odor and acts moderately on the hair and skin has been introduced into considerably wide use as the main ingredient of permanent waving compositions in place of thioglycolic acid having the above drawbacks. Permanent waving compositions consisting mainly of cysteine, nevertheless, are prone to deteriorate during preservation owing to the oxidation of cysteine to cystine which is hardly soluble in water and separates out as crystals to seriously reduce the commercial value of the compositions. During permanent waving, cystine is also likely to separate out in the form of crystals as deposited on the hair or scalp of the customer or on the hands of the hair-dresser, consequently giving a disagreeable feeling. Since crystals of cystine are not removable even when washed with water, the deposition of cystine leads to a reduced work efficiency, possibly leaving white powder on the hair to impart an unattractive appearance to the finished hair. Moreover, cystine, when adhering to the hands of the hair-dresser, especially to crotches of her fingers, becomes firmly bonded to the skin and is not easily removable. Presently this poses an occupational problem pertaining to the hair-dressers.
Attempts have been made to remedy the foregoing drawbacks of permanent waving compositions comprising cysteine as the main ingredient. Published Examined Japanese Patent Application No. 14934/1973, for example, discloses addition of thioglycolic acid to provide a waving composition having improved storage stability. However, thioglycolic acid, which has a peculiar unpleasant odor and acts intensively on the hair and skin as mentioned above, deteriorates the advantages of the composition afforded by cysteine which has a mild action and no objectionable odor. Thus the use of thioglycolic acid should preferably be avoided. Published Unexamined Japanese Patent Application No. 128241/1977 proposes conjoint use of cysteine and N-acetylcysteine to give enhanced stability to the composition during application. The proposal nevertheless has the drawback that sufficient stability, if achievable, requires the use of a relatively large quantity of N-acetylcysteine which is expensive.